The invention relates to recovering data from multiple head video recorders and more particularly, to deskew circuitry for removing overlap in data recovered via multiple head recorder systems or via other data generating sources, where the overlap may occur during an active video display period.
In present and future sophisticated multiple head video recorders such as, for example, helical broadcast videotape recorders (VTR) and future digital videotape recorders (DVTR), multiple heads are used to record and playback video signals at, for example, reduced data rate or reduced tape wrap angle. By way of example only, one possible multiple head configuration includes two heads located 180 degrees apart on a helical scanner device, such that only one head at a time records data during a pass period. The pass period corresponds to one head scan across the tape, and at least two or more pass periods (for example, three pass periods) are required in one field of time. To provide continuous data recording and playback, one head begins recording its respective track on tape while the previous head finishes recording its track on the tape. The data being recorded is switched alternately from one head to the other head during the blanking period of the video signal, whereby the heads record the signal during the active pass period as they successively scan their tracks, thereby providing a continuous recording of the data.
Such an alternate use of heads for recording data can cause an information overlap condition during the subsequent playback operation. The overlap can be created by head misalignment as between different recorders, tape dimensional changes, or during the variable playback speeds used for special playback functions. The overlap may occur during stop or slow motion modes of operation because of the difference between the static and dynamic track lengths when operating in such special modes. Thus, an overlap condition occurs where the data recovered by a second head is advanced in time from its "correct" timing, whereby the beginning of the second data period overlaps the end of a previous data period corresponding to data generated by the previous head. This overlap condition causes playback data to be received simultaneously from both heads during the overlap period instead of being received from one head at a time in alternate succession.
In another multiple head configuration, a dual write head may contain two record gaps and an associated dual read head may contain two playback gaps. In such a system, a single incoming stream of data is split into two electronic data recording paths and is selectively recorded by the two record gaps simultaneously. Likewise, the recorded data then is played back by the two playback gaps simultaneously, and then is reassembled into a single output stream of data corresponding to the initial incoming data. Thus, during playback the system has two active playback electronics functioning continuously. If overlap occurs between successive blocks of data when reading back the split data, there are four active channels of required playback data occurring for only two electronic playback paths.
In a two head system such as mentioned above where it takes three passes to provide one field, two of the three switches between heads in one field occur during the active video display period. It follows that the occurrence of an overlap condition during the active video display period will cause a visible disturbance in the picture. To remedy this undesirable situation, present systems require that any overlap condition may occur only during the blanking period of the video signal. More particularly, the amount of overlap must be limited in its time of occurrence as well as in size to insure that it occurs only during the blanking interval.
The invention removes the requirement that the overlap, if it occurs, must be restricted in time to insure that it occurs only during the blanking period of the video signal when reproducing the recorded signal. The removal of this requirement, in turn, adds flexibility to the playback process in that special functions may be performed by a recorder in the playback mode such as stop or slow motion where overlaps may occur during the active video interval, since the invention insures that any overlap which occurs during the active display period is removed.
To this end, the invention provides for processing data as it is received alternately from multiple heads, using one playback circuit. A deskew processor circuit is placed in a playback processing circuit, preferably as far forward in the circuit as possible to further minimize the amount of multiple channel processing electronics required. The deskew processor circuit includes first and second memory units having respective first and second memories with independent write and read control circuits. The write control circuits alternately load data from the heads into the respective memories at the off-tape data rate, and the read control circuits independently read data from the respective memories and supply the data to a data selector means via respective data buses at a read data reference rate. Thus, the write and read control circuits for the respective memories are synchronized to respective data timing signals such as, for example, tape horizontal wherein however, the read timing signals may be delayed in accordance with detected overlap conditions to remove the overlap. A data selector control means is coupled to receive the respective tape horizontal signals indicative of the data timing as data is played back by the heads, determines the amount of overlap if any exists, and correspondingly determines the amount of delay to generate prior to switching the data selector means in response to the overlap. Thus the delay generated by the data selector control means in one channel inhibits a reading mode in the other channel until the overlap is removed, to thus deskew the data.